Wait for events, and return the output events matching the event
mask. This is level-triggered polling, i.e. if a descriptor continues
to match an event mask, it is again reported the next time wait
is invoked.

There is no guarantee that the list is complete.

It is unspecified how the set reacts on descriptors that became
invalid (i.e. are closed) while being member of the set. The set
implementation is free to silently disregard such descriptors,
or to report them as invalid. It is strongly recommended to
remove descriptors before closing them.

method dispose : unit -> unit

Release any OS resources associated with the pollset. The pollset
remains usable, however, and by invoking any method except
cancel_wait the required OS resources are automatically allocated
again. Only for cancel_wait it is ensured that the disposal
remains in effect.

method cancel_wait : bool -> unit

There is a cancel bit in the pollset, and this method sets it
to the argument value.

When the bit is true, any wait invocation will immediately
time out. It is allowed to set the cancel bit from another thread,
and when it becomes true, a parallel running wait will
see that and timeout. Note that there is no
protection against race conditions: It is possible that wait
returns events that are found at the same time the cancellation
is carried out, i.e. you cannot rely on that the list returned
by wait is empty.

This is the only method of a pollset that is allowed to be called
from a different thread. If not supported on a platform this method
will fail.